An exposure apparatus is one type of precision assembly that is commonly used to transfer images from a mask to a substrate in various manufacturing processes. A typical exposure apparatus usually includes one or more stages or plants for retaining and moving the mask and/or the substrate. One example of an exposure apparatus is a photolithographic machine called a wafer scanner or wafer stepper, which performs one of the many essential steps in the manufacturing process of integrated circuits (ICs). The wafer scanner or stepper includes a reticle stage that retains a reticle, i.e., mask, and a wafer stage that retains a semiconductor wafer, i.e., substrate. During the manufacturing process, a control system generates signals (e.g., voltage or current) that generate forces to drive several actuators that control the position of the reticle stage and/or wafer stage relative to an illumination source and optical assembly with high precision.
As the circuitry on ICs become smaller, the precision required for controlling movement of the stages increases proportionally. In order to meet specifications that are currently on the order of nanometers, control systems require careful design. Precise positioning of the wafer and the reticle relative to the optical assembly is critical to the manufacture of high density, semiconductor wafers.
During stage movement, a stage may experience a positioning error quantified as the difference between an intended or desired trajectory of the stage and an actual trajectory of the stage at a specified time. Errors such as these can occur, for example, because of a lack of complete rigidity in the components of the exposure apparatus or because of periodic vibration disturbances of various mechanical structures. As a result, precision in the manufacture of the semiconductor wafers can be compromised, potentially leading to issues in production quality and throughput.
Attempts to decrease positioning errors generally include the use of feedback control loops and feed-forward based compensation schemes. Additional control techniques such as iterative learning control (ILC) provide options for significantly improving the tracking performance (when compared to only conventional feedback and feed-forward control) of processes or systems that execute the same trajectory, motion or operation repetitively. For example, a control system may repeatedly perform the same movements to image numerous identical ICs on the same semiconductor wafer. In addition, a control system may repeatedly perform the same, repetitive, stage motions as multiple identical wafers are imaged. For these types of systems, feed-forward ILC can be applied to improve system performance by reducing or eliminating repetitive errors.
Even recognizing the accomplishments of these existing control systems in reducing positioning error, there is significant room for improvement in error handling. Along with the ever-present desire to manufacture smaller ICs and other micro-devices comes a requirement for even more precise stage movement with smaller positioning errors. Thus, there is a need for control methods and systems that can improve the accuracy in the positioning of one or more stages of a precision assembly. Further, there is a need for control systems that can accurately adjust the positioning of the wafer stage and/or the reticle stage in an exposure apparatus to produce higher quality semiconductor wafers.